1. Field of the Invention
The present invention relates to a method for manufacturing a substrate of a flat panel display device and, more particularly, to a method for manufacturing a substrate of a flat panel display device with nematic liquid crystal molecules.
2. Description of Related Art
The requirements for a colorful flat panel display device nowadays have a tendency toward large panel size, high resolution, and high image quality. To meet the requirement of high image quality for the flat panel display device, the Uchida Laboratory of Tohoku University developed an optically compensation bend mode (OCB-mode) liquid crystal display device, which has higher response speed and wider viewing angle than the conventional twisted nematic mode (TN-mode) LCD does.
Generally speaking, to increase the response speed of the OCB-mode LCD, the time that the liquid crystal molecules takes to transform from a splay state into a bend state while applying a voltage needs to be shortened. In the conventional method, the substrate of the OCB-mode LCD is subjected to gradation exposure to form the saw-toothed surface with a taper angle more than 60 degrees over the opaque area of the substrate, i.e. the area where the black matrix (BM) of the LCD covers.
As a result, the liquid crystal molecules over the opaque area can keep the bend state when no voltage is applied, and act as the transition nucleus of the liquid crystal molecules over the transparent area (i.e. the pixel area) when a voltage is applied. The transition time of the liquid crystal molecules is therefore shortened. Unfortunately, the mask used in this manufacturing method is expensive, and the steps of this manufacturing method are difficult and complex.
U.S. Pub. Pat. No. 2003/0133065A1 disclosed a method for forming multiple alignment films, such as homeotropic alignment film and homogeneous alignment film, in the OCB-mode LCD by specific photolithography.
With reference to FIG. 1, there is shown a flowchart for manufacturing alignments films according to prior art U.S. Pat. No. 2003/0133065A1. The method is achieved by forming a transparent electrode 102 and an insulating layer 103 over the substrate 101 first (shown in FIG. 1A). The method is subsequently processed by forming a patterned photoresist 120 on the homogeneous alignment film 110, and coating a homeotropic alignment film 111 on the surface of the homogeneous alignment film 110 and photoresist 120 (shown in FIG. 1B). After a part of the homeotropic alignment film 111 is peeled away, the photoresist 120 is removed to form a patterned homeotropic alignment film 111 (shown in FIG. 1C). Finally, the alignment films are subjected to rubbing by a rubbing roller 130 to improve the homogeneous alignment characteristic of the homogeneous alignment film 110.
In other words, this method is achieved by forming the homogeneous alignment film over the pixel area (i.e. the transparent area) functioning as the homogeneous alignment zone, and forming the homeotropic alignment film over the non-display area (i.e. the opaque area) functioning as the homeotropic alignment zone. However, excessive exposure with the mask is still needed in this method. Hence, the manufacturing cost of the OCB-mode LCD is still high.
In addition, the method for forming multiple alignment films in the OCB-mode LCD can also be achieved by forming a homeotropic alignment film first. Then a photoresist is coated on homeotropic alignment film, and defined in the opaque area by photolithography to expose a part of the homeotropic alignment film. After that, the homeotropic alignment film is subjected to multiple rubbings. Therefore, the exposed homeotropic alignment film with low pretilt angle can function as the homogeneous alignment zone, and the homeotropic alignment film covered by the photoresist can function as the homeotropic alignment zone. However, excessive exposure with the mask is still needed in this method. Hence, the manufacturing cost of the OCB-mode LCD is still high. Besides, the problems of rubbing mura (i.e. the image defect of non-uniform display brightness) and residual pieces caused by repeated rubbings can reduce the yield of the LCD.
So far, it can be seen that many problems such as complex processes, and high manufacturing cost need to be solved for the application of OCB-mode LCD. Therefore, it is desirable to provide an improved method to mitigate and/or obviate the aforementioned problems.